The ceremony follows the historic Accession Council in St James’s Palace during which Charles swore an oath to privy counsellors.
A procession made up of 26 men of the 3rd Battalion the Royal Welsh, supported by the Band of the Royal Welsh and accompanied by the regimental mascot, marched from Cardiff City Hall at 11.25am to the castle before the proclamation was read in English and Welsh.
Watch the moment Charles is proclaimed King in Wales:
Taking place at noon, there was a limit of 2,000 people inside the castle in the capital with access granted on a first come first serve basis.
Gates opened at around 10am and closed at 11.15am. Proclamations also took place in other parts of the UK at the same time.
After the readings, 21 rounds were fired before the singing of God Save The King and Hen Wlad Fy Nhadau, Wales’ national anthem.
It was the third time in three days that canon fires were resounded across the city to mark both the Queen’s death and her son Charles’ accession to the throne.
Flags on the castle and council buildings, which were at half-mast, were returned to full-mast on Saturday to coincide with the Proclamation of the new monarch in London. Flags were then returned to half-mast at 1pm on Sunday.
The Senedd was also recalled on Sunday at 3pm to allow members to pay tribute to the Queen, but all other business has been suspended until after the state funeral.
Several main roads through the city centre will be closed between 7am and 2pm, including:
Gorsedd Gardens Road from the junction with Park Place
Museum Avenue from the junction with College Road down to City Hall
King Edward VII Avenue up to the Glamorgan Building
City Hall Road in its entirety
Kingsway from the junction with North Road to the junction with Duke Street
Cowbridge Road East from the junction with Cathedral Road to the junction with Westgate Street
Saunders Road from the junction with St Mary Street
The following roads will be closed in their entirety: Duke Street; Castle Street; High Street; Westgate Street; Quay Street; Guildhall Place and Golate. Where possible access will be maintained for residents and businesses
King Edward VII Avenue; Museum Avenue; City Hall Road; College Road and Gorsedd Gardens Road.